Adsorption And Separation Of CH₄/N₂ By Electroneutral Framework Molecular Sieves

Coalbed methane reserves in our country,due to the complex geological factors of coalbed methane are widely distributed,and the limitation of the mining technology,directly caused most of the coalbed methane in the form of low concentration coal-bed gas emptying,make a lot of resource waste and caused serious greenhouse effect,so from the low concentration of coalbed methane recovery and enrichment of CH₄ supplement gas source,It has the dual significance of recycling resources and reducing greenhouse gas emissions.Due to air mixing in low-concentration coalbed methane,there is a large amount of N₂,which makes its utilization difficult.Therefore,efficient and economical CH₄/N₂ separation process is needed to realize the recovery and enrichment of CH₄ in low-concentration coalbed methane.Pressure swing adsorption separation technology（PSA）has shown great competitive advantages in gas separation industry due to its advantages of convenient start-up and shutdown,high degree of automation and low energy consumption.The key to its efficient operation lies in the development of high-performance adsorbent.Molecular sieve has excellent thermal stability and chemical stability,pore size adjustable,environmental protection and easy regeneration characteristics,is one of the most successful industrial adsorbent materials,and it is still in the rapid development.In our study on CH₄/N₂ separation,we found that the pure silicon ZSM-5 zeolite with electrically neutral skeleton structure has no equilibrium cation and can reduce the quadrupole moment interaction with N₂,thus showing higher CH₄/N₂ adsorption selectivity than traditional low-silicon zeolite adsorbents.In addition to pure silicon molecular sieve,electroneutral framework molecular sieve also includes isoelectronic analogues of pure silicon molecular sieve——aluminum phosphate（Al PO）molecular sieve and trivalent heteroatomic isocrystalline substituted aluminum phosphate（Me Al PO）molecular sieve.In this paper,the adsorption and separation properties of CH₄ and N₂ of three kinds of molecular sieve adsorbents with neutral skeleton structure were studied.ZSM-11 zeolite has similar framework and pore structure to ZSM-5 zeolite,so the pure silicon ZSM-11 zeolite with electrically neutral framework structure also has the potential to be a high CH₄/N₂ adsorption selective adsorbent.Studies on adsorption and separation of CH₄/N₂ by Al PO molecular sieve are rarely reported.Based on the kinetic diameter of 3.8?of CH₄ molecule,four eight-ring pore zeolites,Al PO₄-17,Al PO₄-18,Al PO₄-33 and Ui O-7,were selected.For the third type of Me Al PO zeolites with neutral framework structure,different metal heteroatoms were doped in Al PO₄-33 to change the internal microenvironment of the pores,so as to enhance the interaction between the adsorbed gas and the molecular sieve framework,in order to improve the adsorption and separation performance of CH₄/N₂.The main research contents and conclusions include the following three parts:（1）ZSM-11 molecular sieves with Si/Al of 40,100,200 and 500 were synthesized by hydrothermal method by controlling the Si/Al in the gel.The structure and physical and chemical properties of ZSM-11 zeolites were characterized by powder X-ray diffraction（PXRD）,77 K@N₂ adsorption and desorption test,thermogravimetric analysis and ICP-OES test.The adsorption capacity of CH₄ and N₂ on ZSM-11 zeolite samples with different Si/Al was measured.It was found that the adsorption capacity of CH₄ and N₂ decreased with the increase of Si/Al ratio.The equilibrium selectivity of CH₄/N₂ calculated by ideal adsorption solution theory（IAST）remains unchanged with the increase of Si/Al ratio and is above 3.7,which is better than that of commercial molecular sieve adsorbents.We concluded that with the increase of the Si/Al balance cation content reduce,molecular sieve and CH₄ and N₂ forces also gradually reduced,so the gas adsorption quantity also gradually decreases,and its molecular sieve ZSM-11 for selective of CH₄ is not along with the increase of Si/Al was improved,but the selectivity and adsorption capacity comprehensively the molecular sieve has a good ability of CH₄/N₂ separation.（2）Al PO₄-17,Al PO₄-18,Al PO₄-33 and Ui O-7 molecular sieves with neutral skeleton structure were synthesized by hydrothermal method.The structure and physical and chemical properties of four Al PO zeolites were characterized by PXRD,field emission scanning electron microscopy（SEM）,77 K@N₂ adsorption and desorption test and thermogravimetric analysis.The CH₄/N₂ adsorption and separation properties of Al PO zeolites were also studied.Experimental results have shown that Ui O-7,with the topological type ZON,has the lowest Brunauer–Emmett–Teller（BET）surface area,but exhibits the highest CH₄ adsorption capacity of 0.82 mmol/g and the highest adsorption selectivities of 4.5（50:50,v/v）and 4.7（20:80,v/v）among the four materials.BET surface area has been evaluated from N₂ adsorption and desorption isotherms at 77 K and the average pore sizes of the four materials have been calculated on the basis of a density functional theory（DFT）model.Interestingly,the CH₄adsorption capacity and adsorption selectivity have been found to first increase and then decrease with increasing average pore size of the four Al PO materials（Al PO₄-18:5.6?,Al PO₄-17:6.9?,Ui O-7:7.7?,Al PO₄-33:8.8?）.Compared with the data of activated carbon material that also has no surface adsorption sites,it is found:They were maximized when the average pore size of the adsorbent was close to 7.6?（diameter of CH₄,3.8?）.Theoretical calculations revealed that the sui Tab.pore size of Ui O-7,match better with CH₄ to provide stronger binding energy with CH₄ than other selected materials.And this result is verified by GCMC simulation.Thus,we infer that this is why Ui O-7 shows the highest CH₄ selectivity.Mixed-gas breakthrough experiments and PSA simulations have further proved that Ui O-7 may be used for CH₄/N₂ adsorption separation and indeed is the best choice among the studied Al PO molecular sieves.（3）Five kinds of Ni,Cu,Cr,Eu and Ga Al PO₄-33 molecular sieves were synthesized and characterized by PXRD,77 K@N₂ desorption test and thermogravimetric analysis.The structure and physical and chemical properties of Me Al PO₄-33 molecular sieve were analyzed,and the effect of heteratomic doping on the adsorption performance of pure aluminum phosphate molecular sieve CH₄ and N₂ was studied.The results showed that at 298 K and 1 bar,the CH₄ adsorption capacity of Al PO₄-33 was increased by 9%from 0.75 mmol/g to 0.82mmol/g,and the selectivity was slightly improved from 3.86 to 3.93 by Ga doping.The calculation of adsorption heat shows that the introduction of heteroatoms leads to the increase of adsorption heat and changes the interaction force between molecular sieve channels and gas molecules.The results show that the isocrystalline substituted heteroatom doping can effectively adjust the internal microenvironment of the molecular sieve,thus affecting the adsorption performance of aluminum phosphate molecular sieve CH₄ and N₂.The introduction of Ga heteroatom has a positive effect on the gas adsorption performance of aluminum phosphate molecular sieve CH₄,which is a feasible means to improve the adsorption capacity of molecular sieve CH₄.